1. Field of Invention
The invention relates to the processing of audio signals which are used to drive loudspeakers.
2. Description of the Prior Art
The simplest and most conventional approach to loudspeaker design starts with a single full range driver, most often in the four- to eight-inch diameter range. This type of single drive design cannot produce high fidelity sound, however, since higher frequencies (above approximately eight KHz) are attenuated due to a combination of cone losses, voice coil inductance defects, and other factors. The aforementioned single driver system will exhibit undesired "beaming" of high frequencies due to the narrowness of a radiation pattern. The fundamental resonance of the system is determined by the physical parameters of the driver and the box and usually will be found somewhere between 60 and 200 Hz. Below fundamental resonance the output of the loudspeaker will fall at the rate of 12 dB per octave for a closed box and 18 dB per octave for a ported or drone cone arrangement. The design approach to solve both the high and low frequency problems associated with the single full range driver system is to add additional drivers, smaller drivers for the high frequencies and larger drivers for the low frequencies. Cross-over circuits are employed to restrict the driver signals to frequencies within the operating range of the drivers. Conventional practice usually leads to a boxed 12- to 15-inch driver for the low frequencies with a combination of the driver and box exhibiting a fundamental resonance somewhere between 30 and 60 Hz.
During the past decade, considerable attention has been given to improving the low frequency response of speakers with a view towards extending the response below that which can be obtained with the common unequalized voltage driven closed box. Serious techniques used, however, extend the bass response about 1 octave, at the most, below which the response falls off at a rate of 24 to 30 decibels per octave with accordingly large phase shifts. It is now well recognized that these phase shifts are as detrimental to accurate sound reproduction as are frequency response anomalies.
Another approach to enhanced low frequency performance uses sub-woofer units which cross-over below 100 Hz. These sub-woofer units are very high priced. They are physically large boxes enclosing high-mass drivers, a combination of which is used to move the fundamental resonance as low as possible. The sub-woofer itself employs no unusual techniques, but rather embodies a very direct approach to extending low frequency response by lowering fundamental resonance.
The previously described prior art speaker systems operate primarily above fundamental resonance. Attempts have been made to extend response well below resonance by the use of a closed loop feedback scheme. Commercial embodiments of this scheme use an accelerometer mounted on the speaker cone. The output from the accelerometer is processed to produce a displacement scaled signal which is then compared to the speaker input signal. The resulting error signal is used as negative feedback to the power amplifier for reducing distortion. There are a number of inherent difficulties associated with this scheme since distortion will produce ambiguities and confusion in the drive displacement signal. It is imperative that a feedback scheme use a directly scaled displacement transducer.
A still further approach found in the prior art is the use of a special amplifier employing techniques associated with the use of negative output resistance. In the simple case, negative output resistance can be used to cancel the effects of finite voice coil resistance. In the more complex case, the imposition of a suite of conjugate impedances can be used to nullify the deleterious affects of mechanical and acoustic compedances of the system. While this technique is perhaps the most complete available, it is by necessity very expensive and difficult to implement.
With the exception of the aforementioned accelerometer scheme, none of the prior art devices extend the low frequency response of loudspeaker systems more than an insignificant amount below fundamental resonance. The range below fundamental resonance can be viewed, in some respects, as an uncharted region into which successful audio forays are rarely made without sophisticated and expensive equipment.